The present invention relates to a voltage converter and a method for controlling a voltage converter.
Voltage converters are used today in a variety of different applications. For example, DC converters, also referred to as DC/DC converters, may be used in electric vehicles in order to convert a voltage of a vehicle battery into a voltage suitable for the respective consumers, for example, a drive motor.
Voltage converters may have different topologies. The possible converter topologies are in particular grouped according to the required converter power. For power levels up to approximately 100 watts, for example, so-called forward converters or flyback converters may be used. For power levels above 100 watts, for example, half-bridge voltage converters may be used, and for power levels of approximately 200 watts to above 1000 watts, so-called “zero-voltage switching” full-bridge voltage converters may be used.
For example, US 2013 314 949 A1 describes such a full-bridge voltage converter.
Such full-bridge voltage converters may be driven by means of different switching strategies. One possibility is to drive the full-bridge voltage converter by means of PWM signals, in which the phases are shifted relative to one another.
In the case of full-bridge converters, four switching signals are required, wherein two switching signals are jointly configured in each case in such a way that one of the terminals of the converter coil is connected to a positive supply voltage and one of the terminals of the converter coil is connected to a negative supply voltage. For the sake of simplicity, each such pair of control signals will be referred to below as a PWM signal.
Generally, these PWM signals are provided in each case with a PWM duty cycle of 50%. If these PWM signals are then phase-shifted relative to one another, different supply intervals may be set, in which the converter coil is supplied with electrical energy from one energy source. In the sections in which the “ON” intervals of the two PWM signals do not overlap, the inductive oscillation circuit is in a freewheeling state.
When driving such voltage converters, it is important to detect saturation of the converter transformer, which may occur due to possible component tolerances and a resulting asymmetrical current feed to the converter transformer.
If the converter transformer is driven by asymmetrical currents, it may go into saturation. However, during saturation, the inductance of the converter transformer decreases, and the currents within the converter transformer thus increase for the same voltage. This may result in damage to the converter transformer or the upstream electrical components.
In order to prevent this, the currents which flow into the converter transformer may be measured. For this purpose, a current sense transformer is generally placed at the positive supply line of the full bridge of the voltage converter. However, since a current always flows in the same direction through this current sense transformer, this current sense transformer goes into saturation and can no longer be used for measuring the current.
For this reason, when driving the full bridge, a reset phase is introduced in each case, which switches the supply line of the full bridge at zero current, thus preventing saturation of the current sense transformer.
A block diagram of a voltage converter corresponding to this principle is shown in FIG. 6. The voltage converter has a full bridge with four switches S1 to S4, in the transverse branch of which a transformer T1 is arranged. A measuring transformer T2 is situated in one supply line of the full bridge.
However, it is disadvantageous that due to the reset phase, in which no current is allowed to flow, the control or regulating range for the PWM signals is limited. However, in particular in the case of voltage converters for high power levels, it is desirable to be able to use the entire control or regulating range for the voltage conversion.